This invention relates to high temperature resistant epoxy-based adhesive films.
Polyepoxide resins are monomers or prepolymers that react with curing agents to yield high performance resins. These resins have gained wide acceptance in structural adhesives because of their combination of characteristics such as thermal and chemical resistance, adhesion, and abrasion resistance.
Cured epoxy resins are frequently required to have high glass transition temperatures in order to provide adhesives having structural properties at high temperatures. Examples of methods of achieving high glass transition temperatures in such polyepoxide resins include: preparing resins having a high crosslink density and a high concentration of polar groups as disclosed in U.S. Pat. No. 4,331,582; using epoxy resins compositions in which the epoxy-group containing compound contains a polycyclic structure, such as in U.S. Pat. Nos. 2,902,471; 3,298,998; and 3,332,908; using epoxy resin compositions in which the curing agent or hardener contains a polycyclic structure; and combining a 9,9-bis(aminophenyl)fluorene with an aromatic epoxy resin as described in U.S. Pat. No. 4,684,678.
Although many of these compositions can be cured to resins having a high glass transition temperature, the cured resins typically are highly crosslinked, and are brittle or have a low ductility. One method of improving the ductility of such cured resins is by adding a rubber component or toughening agent to the composition. However, many compositions containing cured epoxy resins having a high glass transition temperature are incompatible with such toughening agents.
In one aspect, the invention provides an adhesive composition comprising a mixture of polyepoxide resins comprising cycloaliphatic-containing polyepoxide resin and aromatic polyepoxide resin and an effective amount of 9,9-bis(3-methyl-4-aminophenyl)fluorene. The mixture of polyepoxide resins has a cycloaliphatic character of greater than 10 weight percent, as defined below. The cycloaliphatic-containing polyepoxide resin is present in the adhesive composition in an amount of at least about 20 to about 80 weight percent, based on the total weight of the polyepoxide resins. The aromatic polyepoxide resin is present in the adhesive composition in an amount of from about 80 to about 20 weight percent, based on the total weight of the polyepoxide resins. In another embodiment, the adhesive composition further comprises a toughening agent.
The adhesive compositions of the invention are useful for providing adhesives that rapidly cure to provide adhesive bonds having both high peel strength and high overlap shear strength at room temperature and at 120xc2x0 C. to 150xc2x0 C. or higher.
The adhesive compositions of the invention contain at least one cycloaliphatic-containing polyepoxide resin. The cycloaliphatic-containing polyepoxide resins have epoxide moieties attached to, that is, pendent from, only aromatic groups and have cycloaliphatic groups between the aromatic groups. A general formula for such compounds is R(Xn)xe2x80x94R1xe2x80x94(R(Xp)xe2x80x94R1)rxe2x80x94R(Xn), where each R is independently at least a divalent aromatic group, each X is an epoxide-containing moiety, each R1 is independently at least a divalent cycloaliphatic group, and p and r are xe2x89xa70 and n is at least 1. Examples of such cycloaliphatic groups include the divalent radicals of dicyclopentadiene, cyclopentadiene, norabornane, decalin, and hydrogenated analogs of naphthalene, anthracene, and biphenyl compounds, and combinations thereof. Examples of aromatic groups include mono- and divalent radicals of benzene, naphthalene, bisphenol-A, bisphenol-F, and biphenyl-type compounds, and combinations thereof. The aromatic groups may be substituted, for example, with alkyl groups on the aromatic rings.
In some embodiments, adhesives and adhesive compositions of the invention contain one or more dicyclopentadiene-containing polyepoxide resins. Such resins are generally prepared from the reaction of dicyclopentadiene phenol resin and epichlorohydrin under basic conditions. The detailed preparation procedure can be found in publicly available literature. Examples of useful dicyclopentadiene-containing polyepoxide resins have the formula: 
wherein n is an integer from 0 to 7 and may be any integer or fraction in between 0 and 7. Epoxide equivalent weights range from about 150 to about 500. Commercially available dicyclopentadiene-based epoxy resins include HP-7200 from Dainippon Ink and Chemicals, Inc., TACTIX(trademark) 71756 and TACTIX(trademark) 556 epoxy resins, available from Vantico, Inc, Brewster, N.Y.
Cycloaliphatic-containing polyepoxide resin is present in the compositions of the invention in an amount of from 20 to 80 weight percent of the total weight of the polyepoxide resins present in the adhesive composition, and may be present in any whole or fractional amount between 20 and 80 weight percent. In other embodiments, the cycloaliphatic-containing polyepoxide resin is present in the compositions of the invention in an amount of from 25 to 75 weight percent of the total weight of the polyepoxide resins present, and any whole or fractional amount between 25 and 75 weight percent.
Suitable aromatic polyepoxide resins include those containing at least two 1,2-cyclic ethers. Such compounds can be aromatic or heteroaromatic, or can comprise combinations thereof. Suitable polyepoxide resins may be solid or liquid at room temperature. Aromatic polyepoxide resin is used in the adhesives and compositions of the invention to increase the Tg of the cured adhesive film and to provide heat resistance. Aromatic polyepoxide resins do not include polyepoxide resins having both aromatic and cycloaliphatic groups.
Compounds containing at least two epoxide groups (i.e., polyepoxides) are preferred. A combination of polyepoxide compounds may be employed, and an epoxide resin having a functionality of less than two may be used in a combination so long as the overall epoxide functionality of the mixture is at least two. The polymeric epoxides include linear polymers having terminal epoxide groups (e.g., the diglycidyl ether of bisphenol-A) and polymers having pendent epoxy groups (e.g., polyglycidyl ethers of phenolic novolak compounds). It is also within the scope of this invention to use a material with functionality in addition to epoxide functionality but which is essentially unreactive with the epoxide functionality, for example, a material containing both epoxide and acrylic functionality.
A wide variety of commercial epoxide resins are available and listed in xe2x80x9cHandbook of Epoxy Resinsxe2x80x9d by Lee and Neville, McGraw Hill Book Company, New York (1967); and in xe2x80x9cEpoxy Resin Technologyxe2x80x9d by P. F. Bruins, John Wiley and Sons, New York (1968); and in xe2x80x9cEpoxy Resins: Chemistry and Technologyxe2x80x9d, 2nd Editionxe2x80x9d by C. A. May, Ed., Marcel Dekker, Inc. New York (1988). Aromatic polyepoxides (i.e., compounds containing at least one aromatic ring structure, e.g., a benzene ring, and at least two epoxide groups) that can be used in the present invention include the polyglycidyl ethers of polyhydric phenols, such as Bisphenol-A or Bisphenol-F type resins and their derivatives, aromatic polyglycidyl amines (e.g., polyglycidyl amines of benzenamines, benzene diamines, naphthylenamines, or naphthylene diamines), polyglycidyl ethers of phenol formaldehyde resole or novolak resins; resorcinol diglycidyl ether; polyglycidyl derivatives of fluorene-type resins; and glycidyl esters of aromatic carboxylic acids, e.g., phthalic acid diglycidyl ester, isophthalic acid diglycidyl ester, trimellitic acid triglycidyl ester, and pyromellitic acid tetraglycidyl ester, and mixtures thereof.
Preferred aromatic polyepoxides are the polyglycidyl ethers of polyhydric phenols, such as the series of diglycidyl ethers of Bisphenol-A, commercially available from Resolution Performance Products, Houston, Tex., for example, under the trade designations xe2x80x9cEPON 828xe2x80x9d and xe2x80x9cEPON 1001Fxe2x80x9d and the series of diglycidyl ethers of Bisphenol-A and Bisphenol F and their blends, commercially available from Resolution Performance Products, Pernis, The Netherlands, for example, under the trade designations xe2x80x9cEpikote 232xe2x80x9d and xe2x80x9cEpikote 1001xe2x80x9d. Other useful commercially available aromatic epoxides include the xe2x80x9cDERxe2x80x9d series of Bisphenol epoxides and xe2x80x9cDENxe2x80x9d series of epoxy novolak resins, available from Dow Chemical, Midland, Mich.; diglycidyl ether of fluorene Bisphenol, available from Resolution Performance Products, Houston, Tex., under the trade designation xe2x80x9cEPON HPT Resin 1079xe2x80x9d; a triglycidyl derivative of p-aminophenol, commercially available from Ciba Performance Polymers, Brewster, N.Y., under the trade designation xe2x80x9cMY 0500xe2x80x9d; a tetraglycidyl derivative of methylene dianiline, commercially available from Ciba Performance Polymers, Brewster, N.Y., under the trade designation xe2x80x9cMY 720xe2x80x9d; and a polyfunctional aromatic epoxide resin commercially available from Resolution Performance Products under the trade designation xe2x80x9cEPON SU-8.xe2x80x9d Flame retardant epoxides may also be used, for example, the flame retardant brominated Bisphenol-A diglycidyl ether, commercially available from Dow Chemical, Midland, Mich., under the trade designation xe2x80x9cDER 580xe2x80x9d. The term xe2x80x9cderivativexe2x80x9d as used herein with reference to thermosetting materials refers to a base molecule with additional substituents that do not interfere with the thermosetting curing reaction of the base molecule.
Aromatic polyepoxide resin is present in the compositions of the invention in an amount of from 80 to 20 weight percent of the total weight of the polyepoxide resins present in the adhesive composition, and may be present in any whole or fractional amount between 80 and 20 weight percent. In other embodiments, the aromatic polyepoxide resin is present in the compositions of the invention in an amount of from 75 to 25 weight percent of the total weight of the polyepoxide resins present, and any whole or fractional amount between 75 and 25 weight percent.
The combination of cycloaliphatic-containing polyepoxide and aromatic polyepoxide resins have a cycloaliphatic character of greater than 10 weight percent. In one embodiment, the cycloaliphatic character is at least 12 weight percent. In another embodiment, the cycloaliphatic character is at least 13.5 weight percent. In another embodiment, the combined polyepoxide resins have a cycloaliphatic character of not more than about 60 weight percent. In another embodiment, the combined polyepoxide resins have a cycloaliphatic character of not more than about 55 weight percent. In another embodiment, the combined polyepoxide resins have a cycloaliphatic character of not more than about 40 weight percent. In other embodiments, the combined polyepoxide resins have a cycloaliphatic character of from greater than 10 weight percent to about 60 weight percent; at least 12 weight percent to about 55 weight percent; and from about 13.5 weight percent to about 40.5 weight percent, and may be any whole or fractional weight percent in between 12 and 60 weight percent.
xe2x80x9cWeight percent cycloaliphatic characterxe2x80x9d is determined by calculating the weight percent cycloaliphatic groups of the cycloaliphatic-containing polyepoxide, excluding the epoxide-containing moieties attached to the aromatic groups. The value, expressed as a percentage, is then multiplied by the weight percent of cycloaliphatic-containing polyepoxide present in the total amount of polyepoxide resin present in the adhesive composition. For example, if n=1 in the above formula 1, the percent cycloaliphatic character of the cycloaliphatic-containing resin of the formula is 54% (benzene=75.1+76.1+76.1; dicyclopentadiene=134.2; % cycloaliphatic character=268.4/(268.4+227.3)=0.54). If the total amount of cycloaliphatic-containing polyepoxide resins present is 25 weight percent of the total amount of polyepoxide resin, then the polyepoxide resin present in the adhesive composition would have a cycloaliphatic character of 13.5 weight percent (0.54xc3x9725).
The curative for the adhesive films and compositions of the invention is 9,9-bis(3-methyl-4-aminophenyl)fluorene (o-TBAF). o-TBAF is present in the compositions in an effective amount. An xe2x80x9ceffective amountxe2x80x9d of o-TBAF is that amount which cures or crosslinks the polyepoxide resins.
The curative is used in the adhesive films and compositions of the invention in a stoichiometric ratio of 0.8 to 1.7 amino hydrogen (NH) equivalents per epoxide equivalent. In one embodiment, a stoichiometric ratio of 1.0 to 1.65 amino hydrogen equivalents per epoxide equivalent is employed. In another embodiment, a stoichiometric ratio of 1.25 to 1.65 amino hydrogen equivalents per epoxide equivalent is used.
The adhesives and adhesive compositions of the invention may contain one or more tougheners or toughening agents. The toughening agent can be introduced as a latex of dispersed or synthetic rubber as is disclosed in U.S. Pat. No. 3,316,195 or a graded rubber or core shell rubber particle as.disclosed in U.S. Pat. Nos. 3,833,683; 3,856,883; and 3,864,426. The toughening agent can also be introduced into the epoxy resin composition by dissolving reactive elastomers into the epoxy resin which phase-separate during curing. The technique is exemplified by U.S. Pat. Nos. 4,107,116 and 3,894,112. A detailed description of the use of toughening agents in epoxy resin is to be found in the Advances in Chemistry Series 208 titled xe2x80x9cRubbery-Modified Thermoset Resinsxe2x80x9d edited by C. K. Riew and J. K. Gillham, American Chemical Society, Washington, 1984. One specific toughening agent is the insoluble in situ polymerized elastomeric particles that are formed from amine terminated polyethers, for example, diprimary amine endcapped poly(tetramethyleneoxides). Other examples include amine-terminated butadiene/nitrile rubbers, carboxyl-terminated butadiene/nitrile rubbers, and core shell materials.
The adhesive compositions of the invention may contain from about 3 to about 20 weight percent of the total weight of the composition of toughening agent. In other embodiments, the adhesive compositions of the invention may contain from about 3 to about 10 weight percent toughening agent based on the total weight of the composition.
Various other adjuvants can also be added the compositions of the invention to alter the characteristics of the cured adhesive. Useful adjutants include fumed silica, pigments, silica, alumina, magnesium sulfate, calcium sulfate, bentonite, glass beads, glass bubbles, and organic and inorganic fibers. Amounts of up to about 80 weight percent of the total weight of the composition adjuvant can be used.
The adhesive compositions of the invention are generally useful for bonding substrate where both high peel strength and high shear strength at high temperatures are desired.
Overlap Shear Test
The shear strength of the adhesive films of the present invention was determined by bonding two aluminum substrates together using the adhesive film and measuring the shear strength of the resulting construction. More specifically, overlap shear strength was determined according to ASTM D-1002 with the following modifications. The adherends were 2024-T3 bare aluminum panels measuring 7 inches longxc3x974 inches widexc3x970.063 inches thick (178 mmxc3x97102 mmxc3x971.60 mm).
The panels were prepared in the following manner. The panels were first degreased by immersing in alkaline degreaser (xe2x80x9cOakite Aluminum Cleaner 164xe2x80x9d, Oakite Products Inc., Berkeley Heights, N.J.) at about 88xc2x0 C. for about 10 minutes, followed by rinsing with tap water. The degreased panels were then oxidized by immersing them in a 68xc2x0 C. bath of concentrated sulfuric acid, sodium dichromate and water for about 10 minutes (this is known as Forest Products Laboratories Etch System or FPL Etch System) then rinsing with tap water. This was designated as xe2x80x9cSurface Prep 1xe2x80x9d.
In some cases, the panels were further treated as follows. The etched panels were anodized by immersion in phosphoric acid at 22xc2x0 C. with an applied voltage of 15 Volts for 20-25 minutes, followed by rinsing with tap water (test for water break), air drying for 10 minutes at room temperature, then oven drying in a forced air oven at 66xc2x0 C. for 10 minutes. The resulting anodized aluminum panels were immediately primed within 24 hours of treatment. The anodized panels were primed with a corrosion inhibiting primer for aluminum (3M(trademark) Scotch-Weld(trademark) Structural Adhesive Primer EC-3983, obtained from Minnesota Mining and Manufacturing Company, St. Paul, Minn.) according to the manufacturer""s instructions to give a dried primer thickness of between 0.00010 and 0.00020 inches (2.6 to 5.2 micrometers). The complete surface conditioning procedure was designated xe2x80x9cSurface Prep 2xe2x80x9d.
The primed panels were bonded to one another in an overlapping relationship along their lengthwise dimension using a 15.9 mm wide strip of adhesive film. After removing the liner from one side, the scrim-supported film was applied to the first adherend by hand using a small rubber roller in such a manner as to exclude entrapped air and insure intimate contact between the exposed adhesive and the substrate. After removing the second liner, the second adherend was placed in contact with the exposed adhesive surface to give an assembly with an overlap area of 0.5 inches (12.7 mm). The resulting assembly was fastened together using tape and cured in an autoclave in the following manner. After applying a vacuum to reduce the pressure to 28-30 inches (Hg), about 15 pounds per square inch (psi) (103 kPa) pressure was applied and the temperature of the autoclave was heated from room temperature (68xc2x0 F. to 77xc2x0 F. (20xc2x0 C. to 25xc2x0 C.)) to 350xc2x0 F. (177xc2x0 C.) at a rate of 5xc2x0 F./min. (2.8xc2x0 C./min.). The vacuum was released when the pressure reached about 15 psi (69 kPa). The final temperature and pressure were maintained for 60 minutes before cooling to about 25xc2x0 C. The bonded panels were sawn across their width into 1 inch (2.54 cm) wide strips and evaluated for overlap shear strength in accordance with ASTM D-1002 using a grip separation rate of 0.05 inches/minute (1.3 millimeters/minute) using a tensile tester. Testing was conducted at two different test temperatures (room temperature and 177xc2x0 C.). Samples tested at the elevated temperature were equilibrated for between 10 and 15 minutes prior to testing.
Floating Roller Peel Strength Test
Panels of 2024-T3 bare aluminum measuring 8 inches longxc3x973 inches widexc3x970.063 inches thick (20.3xc3x977.6xc3x970.16 centimeters), and 10 inches longxc3x973 inches widexc3x970.025 inches thick (25.4xc3x977.6xc3x970.064 centimeters), were prepared for testing as described above in xe2x80x9cOverlap Shear Testxe2x80x9d. The primed panels were bonded together using the same film adhesive and cure cycle employed for the overlap shear samples, then evaluated for floating roller peel strength in accordance with ASTM D-3167-76 with the following modification. Test strips measuring 0.5 inch (12.7 cm) wide were cut along the lengthwise direction of the bonded aluminum panels. A tensile testing machine operated at a rate of 12 inches/minute (30.5 cm/minute) was used to peel the thinner substrate from the thicker one, and the results normalized to a width of 1 inch.